Integrated circuits are employed in most of the digital appliances utilized on a daily basis by consumers. Integrated circuits may refer to an electronic circuit built onto a single piece of substrate, a chip, and enclosed within a package. Integrated circuits generally perform a desired function or a plurality of functions.
A spiral inductor is commonly utilized in radio-frequency (RF) circuits, voltage-controlled oscillators, low-noise amplifiers and passive element filters. Performance of RF circuits, voltage-controlled oscillators, low-noise amplifiers and passive element filters are found to benefit from a higher quality factor inductor. In some silicon semiconductor integrated circuits, especially those used in flip chip applications, there are problems associated with the traditional spiral inductor configuration.
Referring to FIG. 1, an integrated circuit 100 known to the art is shown. Integrated circuit 100 may include a substrate 110 and a plurality of metal layers 120–150 and an inductor (not shown). If the spiral inductor is placed in the middle of the metallization stack of the integrated circuit 100, such as between metal layer 130 and metal layer 140, a high capacitance associated with the adjacent metal layers may reduce performance of the inductor. Specifically, this capacitance reduces the self resonant frequency and quality factor of the inductor.
Referring to FIG. 2, an integrated circuit 200 in a flip-chip configuration known to the art is shown. The integrated circuit 200 may include a substrate 210, a plurality of metal layers 220–250, underfill material 260 and a package substrate 270. A conventional solution to the problem of capacitance associated with metal layers 220–250 adjacent to the inductor is to place the spiral inductor on top of the metal stack of the integrated circuit 200. For example, the spiral inductor may be placed on the metal layer 250 in proximity to the underfill material 260. However, the electromagnetic field lines of the inductor interact with the underfill material 260 between the chip and the package substrate 270. The underfill material 260 has varying and often poor microwave properties which adversely affect the quality factor of the spiral inductor. Consequently, an inductor that provides a high quality factor within an integrated circuit environment is necessary.